1. Field of the Invention
The present invention relates to a circuit used to write servo onto disks of a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Generally, there are two ways to record data, horizontal recording and perpendicular recording. With horizontal recording the disk is magnetized parallel with the disk surface. In vertical recording the disk is magnetize in a direction perpendicular to the disk surface.
Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
Information is typically stored on radial tracks that extend across the surface of each disk. Each track is typically divided into a number of segments or sectors. The voice coil motor and actuator arm can move the heads to different tracks of the disks.
FIG. 1 shows a typical track that has a number of fields associated with each sector. A sector may include an automatic gain control (“AGC”) field 1 that is used to adjust the strength of the read signal, a sync field 2 to establish a timing reference for the circuits of the drive, and ID 3 and Gray Code 4 fields to provide sector and track identification, respectively.
Each sector may have also a servo field 5 located adjacent to a data field 6. The servo field 5 contains a plurality of servo bits A, B, C and D that are read and used to position the head 7 relative to the track.
The fields 1-5 are written onto the disk surfaces during the manufacturing process of the disk drive. These fields are typically written with a servo writer. The servo tracks are sometimes written using a number of spiral servo tracks initially written onto the disks. FIG. 2 shows an example of a number of spiral servo tracks written onto a disk. Using spiral servo tracks is sometimes referred to as an Ammonite servo write process. The spiral servo tracks are used to write the final radial servo tracks that are utilized during the normal operation of the disk drive.
As shown in FIG. 2, the radial start point of each spiral track may vary. This variation in start points may be caused by system vibration, servo track writer positioning errors and other factors. The variation of the start points will vary the location of the spiral tracks and ultimately create inaccuracies in the final servo pattern. Inaccurate servo can lower the density and degrade the performance of the drive. It would be desirable to improve the accuracy of spiral servo tracks used to write radial servo patterns.
One approach to achieving a common spiral track start point has been to include a clock track that contains a circumferential index, and a band of conventional servo tracks that include a radial index. The clock track is read by a clock head of a servo writer. The reference tracks are read by the heads of the drive. A spiral servo track is written when the circumferential index and the radial index are both detected.
The clock track contains a clock signal. For horizontal recording the clock signal has the form of a sine wave. At low frequencies, perpendicularly recorded clock tracks provide a square clock signal. The square clock signal is differentiated by a differentiator circuit to detect missing or extra pulses. It has been found that signal ringing, undershoot and overshoot may occur when reading a square clock signal. The ringing, undershoot and overshoot may create false signals and inaccurate timing data. It would be desirable to eliminate the signal ringing, undershoot and overshoot that occurs when reading a perpendicularly recorded clock signal.